1. Field of the Invention
The present invention relates to a method for producing a silicon wafer to be used for production of semiconductor devices and, more particularly, to a method for producing a silicon wafer for epitaxial substrates and a method for producing an epitaxial substrate.
2. Description of the Related Art
Recent years have seen a remarkable improvement in the performance of semiconductor devices. High-performance semiconductor devices need a high-quality silicon substrate (or silicon wafer). Better quality is especially required of epitaxial substrates to be used for production of advanced discrete devices represented by power MOSFET. In general, discrete devices are designed such that current flows in the direction from the front side to the reverse side of the silicon wafer. This requires that current in the forward direction should increase according as devices improve in performance. To meet this requirement, it is necessary to reduce resistance components and hence to increase the concentration of impurities in epitaxial substrates.
Production of epitaxial substrates employs epitaxial growth process, which is one kind of vapor phase growth process. This process starts with preparing a silicon wafer for epitaxial substrate by highly doping an impurity, such as boron (B), up to about 1E19 atoms/cm3. (This silicon wafer will be referred to as a silicon wafer for substrate hereinafter.) The silicon wafer for substrate is placed in a reaction furnace in which the atmosphere is kept at a predetermined pressure. It is then heated in the reaction furnace which is being fed with a source gas incorporated with a dopant gas containing phosphorus (P) or boron (B) as an impurity. The heating temperature is 1000° C. or above. This heating causes the source gas to undergo thermal decomposition or hydrogen reduction on the surface of the silicon wafer for substrate. This reaction causes a silicon single-crystal film, which is 1 to 200 μm thick and contains an impurity in a concentration of about 1E14 to 1E17 atoms/cm3, to grow on the surface of the silicon wafer for substrate.
The thus produced epitaxial substrate is used to form desired devices thereon. In this way there is obtained the high-performance discrete device mentioned above.
The disadvantage of the epitaxial growth method that is applied to a highly doped silicon wafer for substrate is that the profile of impurity concentration is poor or the in-plane uniformity of impurity concentration is poor due to autodoping into the single-crystal silicon film to be grown.
Autodoping denotes a phenomenon that when the silicon wafer for substrate is heated at the time of epitaxial growth, boron diffuses outward into the atmosphere from the surface of the silicon wafer for substrate and diffused boron is captured by the single-crystal silicon film which is growing so that the concentration of impurity fluctuates. The consequence is that the profile of impurity in the epitaxial substrate is uneven, which presents difficulties in forming the semiconductor devices with desired characteristics.
Thus, several methods have been proposed to prevent autodoping. One of them consists of previously forming a silicon oxide film by CVD (Chemical Vapor Deposition) on the reverse side of the silicon wafer for substrate, thereby preventing the impurity from diffusing outward. (See Japanese Patent Unexamined Publication No. 10-106955.) Another is intended to perform hydrogen treatment on the highly boron-doped silicon wafer for substrate, thereby reducing the boron concentration in the surface layer. (See Japanese Patent Unexamined Publication No. 9-199380.)
Unfortunately, the known methods mentioned above have some problems. For example, as shown in Japanese Patent Unexamined Publication No. 10-106955, the first one causes anomalous silicon growth to occur on the silicon oxide film at the bevel part when epitaxial growth is performed on the silicon wafer for substrate, with its reverse side coated with a silicon oxide film extending to the bevel part. As the result, the silicon oxide film peels off to give rise to particles that contaminate the inside of the reaction furnace.
Another disadvantage is that when a single-crystal silicon film with a low impurity concentration is grown on the silicon wafer for substrate which has a high concentration of impurity in the surface layer, misfit dislocation tends to occur in the single-crystal silicon film. This problem is due to the difference in lattice constant that results from difference in impurity concentration between the surface of the silicon wafer for substrate and the single-crystal silicon film to be grown. To address this problem, it is necessary to lower the boron concentration in the surface layer of the silicon wafer for substrate. A means for solution is disclosed in Japanese Patent Unexamined Publication No. 9-199380.
According to this Publication, the substrate having a concentration of 3E18 atoms/cm3 is heated at 1200° C. for 2 hours in an atmosphere of 50% hydrogen. However, substrates having a higher concentration need heat treatment longer than 2 hours if they are to produce the same effect as mentioned above. Unfortunately, heat treatment longer than 2 hours adversely affects the durability of the quartz furnace for hydrogen treatment.